Interesting one this OP - I was going to say you will get a skewed result as we are all members of an enthusiasts forum - but then it made me think, did all people join such a forum for the same reason as me.
I joined for the DIY and to learn what the consensus is amongst a non biased (product/corp/paid sense) collective on the best parts and ways of doing things. Others may join for different reasons. I know the cool pic thread is almost reason enough on its own
My personal take is - DIY up until a point my time becomes more valuable than the cost of the work or the specialist tools (that would be used once) negate the saving. Luckily i am very time rich but if I had kids I and we relied in the car then things would be very different.
Side note as you mention risk - A friend once said he thought i was mad for doing my own brake pads (I know, I know) because he wouldn`t trust himself - I asked why he trusted someone he didn`t know, that's under pressure to get a job done in a time frame, and, you have no control on the experience of that person. Now IMO you cant get a brake pad change wrong (easily) and not notice pretty much instantly but the point is valid.